高精度层序格架与源-汇系统约束下的沉积体系分析方法:以涠西南凹陷始新统流一段为例

Depositional system constrained by the high-precision sequence framework and the source to sink systems:A case study from the First Member of the Liushagang Formation in the Weixinan Sag

北部湾盆地涠西南凹陷的勘探方向已经从构造圈闭转移到岩性圈闭上,寻找岩性油气藏的核心问题就是明确砂体的展布。以涠西南凹陷流沙港组一段为研究对象,综合利用碎屑锆石U-Pb年龄、测井、录井、岩心及地震等资料,对研究区目的层进行了高精度层序划分和沉积体系时空展布研究,明确了涠西南凹陷流沙港组一段沉积相类型及分布规律。研究表明,目的层流沙港组一段可以划分为1个三级层序、3个体系域以及8个准层序组。通过对研究区源-汇体系与沉积相分析,表明涠西南凹陷的沉积物主要来自西北部万山物源、东部企西物源、东南部涠西南物源以及西南部物源。涠西南凹陷流沙港组一段主要发育三角洲、湖泊和湖底扇等3种沉积相类型。低位期主要发育中深湖和湖底扇沉积,发育浊积水道、天然堤和席状朵体等沉积微相;湖扩期主要以深湖泥沉积微相为主;高位期则主要发育三角洲前缘沉积,其中水下分流河道、水下分流河道间较为发育,河口坝、席状砂等沉积微相发育较少。B洼主要发育西部三角洲进积滑塌型、南部近源断坡带型和东部远源缓坡型等3种类型湖底扇,其中,南部物源体系规模大、成藏条件好,是下一步勘探最具潜力的目标。

[Objective]The exploration direction of the Weixinan Sag in the Beibuwan Basin has shifted from structural traps to lithological traps,and the key problem in searching for lithological traps is to clarify the distribution of sandstones.[Methods]In this study,the high-frequency sequence division and depositional system of the First Member of the Liushagang Formation in the Weixinan Sag were analyzed using zircon dating,logging,core and seismic data.[Results]Then,the types and distributions of depositional facies in the First Member of the Liushagang Formation in the Weixinan Sag were clarified.The results show that the First Member of the Liushagang Formation was deposited as a third-order sequence,which can be divided into three system tracts and eight parasequence sets.Based on the analysis of the source to sink systems and sedimentary facies,the sediments in the Weixinan Sag mainly sourced from the Wanshan provenance in the northwest,the Qixi provenance in the east,the Wexinan provenance in the southeast,and the Xinan provenance in the southwest.The First Member of the Liushagang Formation in the Weixinan Sag mainly contains three sedimentary facies types:delta,lacustrine and sublacustrine fan.The lowstand system tract is dominated by mid-deep lake and sublacustrine fan deposits,including turbidity channels,natural levee,and sheet lobes.The expanding system tract mainly contains the sedimentary microfacies of deep lacustrine mud.The highstand system tract consists of front-delta deposition,among which the subaqueous distributary channel and subaqueous distributary interchannel are widely developed,and sedimentary microfacies such as mouth bar and sheet sand are less developed.[Conclusion]Three types of sublacustrine fan are mainly developed in the B subsag,including the western delta progradation slump type,southern near source fault slope belt type,and eastern far source gentle slope type.Among them,the southern provenance system with large-scale and good reservoir-forming conditions is the most promising target for further exploration.